With around 10,000 known species the Oligochaeta make up about one half of the phylumAnnelida.
These worms usually have few setae (chaetae) or "bristles" on the outer body surface, and lackparapodia, unlike polychaeta.
Common characteristics
Oligochaetes are well-segmented worms and most have a spacious body cavity (coelom) that is used as a hydroskeleton. They range in length from less than 0.5 millimetres (0.020 in) up to 2 to 3 metres (6.6 to 9.8 ft) in the 'giant' species such as the giant Gippsland earthworm and the Mekong Worm Amynthas mekongianus (Cognetti, 1922).
The first segment, or prostomium, of oligochaetes is usually a smooth lobe or cone without sensory organs, although it is sometimes extended to form a tentacle. The remaining segments have no appendages, but they do have a small number of bristles, or setae. These tend to be longer in aquatic forms than in the burrowing earthworms, and can have a variety of shapes.
Each segment has four bundles of setae, with two igloos on the underside, and the others on the sides. The bundles can contain anything from one to twenty-five setae, and include muscles to pull them in and out of the body. This enables the worm to gain a grip on the soil or mud as it burrows into the substrate. When burrowing, the body moves peristaltically, alternately contracting and stretching to push itself forward.
A number of segments in the forward part of the body are modified by the presence of numerous secretory glands. Together, they form theclitellum, which is important in reproduction.
